Two strains of mice, HSFS/N(susceptible) and HSFR/N(resistant), have been developed by selectivity breeding for their ability to be sensitized to histamine by pertussis vaccine. The responses after more than 30 generations of selection indicate that susceptibility to sensitization to histamine is a heritable trait. The HSFS/N and HSFR/N strains have been shown to differ widely in their susceptibility to the lethal effects of histamine as measured by the median histamine-sensitizing dose. Assays for 12 biochemical markers in the HSFS/N and HSFR/N strains indicate that they are isogenic lines. Both strains were homozygous for all loci tested, as would be expected after 26 generations of sib-matings. The two strains carry the same alleles at all loci except for hemoglobin b-chain (Hbb) and malic enzyme (Mod-1). HSFS/N carries the d-allele of Hbb and the a-allele for Mod-1; HSFR/N carries the s- and b-alleles for Hbb and Mod-1, respectively. Once a genetic profile is established it is possible to distinguish one strain from others. Since genetic contamination is a constant threat in an animal facility that houses more than one strain of a particular species, the genetic integrity of an animal colony can be monitored. In addition to their possible use to evaluate the potency and toxicity of pertussis vaccines, the histamine susceptible and histamine resistant strains will be used in pharmacologic studies to examine the mechanism of action of LPF (HSF) in lowering the seizure thresholds to various intermediates. Now that purified pertussis toxin (PT), the agent believed to be responsible for the HSF (LPF) response, is available we are comparing the strains' response to PT and whole cell vaccine. Final characterization of the HSFS/N and HSFR/N strains awaits the breeding of sufficient animals to complete the necessary laboratory tests.